This invention relates to a wavelength selective optical data storage system.
FIG. 1 shows an optical wavelength division multiplex recording and reproducing apparatus and FIGS. 2A, 2B and 2C show the wavelength spectrum fo the recording medium, which were proposed previously in the Japanese Patent Publication No. 51355/1983. A diverging beam emitted from light source 10 is collimated by collimator lens 12. The parallel beam from lens 12 passes through optical deflector 13 and is focused by objective lens 14 on memory element 16 of recording medium 15 which has a function of wavelength (frequency) selective optical data storage, namely, which exhibits inhomogeneous absorption line broadening and undergoes a photo induced reaction upon exposure to light. One of elements 16 is selected by deflecting the beam through deflector 13. The beam transmitted through element 16 is detected by photodetector 17.
The principle of wavelength selective optical data storage is described below with reference to FIGS. 2A. 2B and 2C. FIG. 2A shows absorption spectrum (inhomogeneous absorption line broadening) of element 16 on which no recording is made. It is understood from FIG. 2A that the light beam, such as a laser, with broad range of wavelength is absorbed by element 16. When the beam of specific wavelength with intensity spectrum as shown by the dotted line in FIG. 2A is incident on element 16, a hole is created or burned at the corresponding wavelength position on absorption spectrum as shown in FIG. 2B. The hole means that a data bit "1" is recorded in the wavelength and therefore a data bit "0" is expressed with the non-hole portion.
Namely, data bits are stored by selective photo induced reactions induced by a narrow band light beam at specific wavelength positions within the broad inhomogeneous line.
Any hole can be created, in other words a data bit "1" can be recorded, by adjusting the wavelength of light source 10 to that of the recording hole and making the light intensity high at the level necessary for recording.
The beam emitted from source 10 is scanned or tuned with respect to wavelength by wavelength controller 11 in a recording wavelength band of an upper limit A to a lower limit B with a constant intensity to reproduce the recorded data. Since the absorptive power of the element is lowered at the wavelength of the recorded hole as shown in FIG. 2B, the output of photodetector 17 has the spectrum as shown in FIG. 2C. Although FIG. 2C shows a wavelength spectrum, photodetector 17 outputs a time series signal corresponding to the spectrum when the wavelength is scanned at a constant rate.
Since known optical wavelength selective optical data storage system is comprised as mentioned above, there is provided no detecting means detecting the relative position between memory element 16 and a beam spot focused thereon. Accordingly it was difficult to position the beam spot precisely on the selected memory element and to perform high density recording.